As a cylinder or the like to be used in a cask or a large size pressing machine for containing and transferring and temporarily storing used nuclear fuel generated from a nuclear reactor, a container in which height, diameter and the like of its drum reach several meters is used. Containers having a wall that is several dozens centimeter in thickness have been suggested from a viewpoint of shielding from y rays or high pressure resistance. A cask for containing and transporting and temporarily storing used nuclear fuel will be exemplified and there will be explained below a conventional container which has been used for these applications.
FIG. 29 is a sectional view showing one example of the conventional cask. The cask 500 is composed of a container 501 which is formed with a body section 501a and a bottom section 501b made of stainless or carbon steel, a basket 502 for used containing nuclear fuel aggregate which is arranged in the container 501, and a neutron shielding body 503 provided on an outer periphery of the container 501. The neutron shielding body 503 is charged into a space between an outer drum 504 and the container 501, and a plurality of heat transfer fins (not shown) are provided between the container 501 and the outer drum 504. As the basket 502, a material to which boron having neutron absorbing ability is added is used.
The bottom plate 501b made of stainless or carbon steel is welded with tungsten-inert gas (TIG welding) or welded with submerged-arc (SAW welding) to the container 501. A neutron shielding material 506 is sealed into the bottom plate 501b. Moreover, a primary cover 507 and a secondary cover 508 are attached to an upper section of the container 501 by bolts. A neutron shielding material 509 is sealed into the secondary cover 508.
γ rays generated from used nuclear fuel aggregate are shielded by the body section 501, the bottom plate 501b, the primary cover 507 and the secondary cover 508. Moreover, neutron is shielded by the neutron shielding material 503 provided on the outer periphery of the container 501, the bottom plate 501b, the neutron shielding material 506 sealed into the secondary cover 508, and the secondary cover 508. A degradation heat of the used fuel aggregate is transmitted from the container 501 via the heat transfer fins to the outer drum 504 and is radiated to the outside therefrom.
Next, how a container having a bottom (“bottomed container”) for the cask shown in FIG. 29 is manufactured will be explained below. FIGS. 30(a) through 30(e) are explanatory diagrams showing one example of the method of manufacturing the bottomed container of the cask shown in FIG. 29. As shown in FIG. 30(a), a metal billet 61 which is cogged into a predetermined dimension is upset onto an anvil with bore and is bored by a punch 63. As shown in FIGS. 30(b) and 30(c), a mandrel 65 is inserted through a hole 64 of the metal billet 61 and while it is being rotated, the hole 64 is widened by a hammer 66. As shown in FIG. 30(d), the mandrel 65 is replaced by a large-diameter mandrel 67 and hollow cogging is carried out by a hammer 68. As a result, the metal billet 61 is thinned so that a cylindrical body is formed (FIG. 30(e)).
FIGS. 31(a), 31(a′), 31(b) and 31(b′) are explanatory diagrams showing the method of manufacturing the bottomed container according to an Erhardt boring method. This method is for pushing a punch 410 into a metal billet 200 put into a container so as to form the metal billet 200 into a cylindrical shape. This metal billet 200 has a rectangular section, and its diagonal length is equal with an inner diameter of a body section 300 of the container. Moreover, since the section of the metal billet 200 is rectangular, spaces 350 exist between the metal billet 200 and the container 200 (FIG. 31(a′)). When the metal billet 200 is upset into the body section 300 of the container and the punch 410 is pushed into a center axis of the metal billet 200, metal flow occurs due to metal swelling function of the punch 410. While this metal flow is filling the spaces 350 and a part of the metal flow is rising in the body section 300 of the container, and the metal billet 200 is formed into the cylindrical form (FIG. 31(b)).
In addition, the bottomed container of the cask can be manufactured also by a backward extrusion pressing method (not shown). In the backward extrusion pressing method, after the metal billet having a circular section substantially equal with the inner diameter of the container is upset into the container, metal flow is generated between the punch and the container by a compressive force of the punch pressurizing along the center axis of the metal billet. While the metal is being raised to a backward direction, the metal billet is formed into a long cylindrical form.
After the cylindrical body section 501a is formed by one of the above-mentioned methods, the bottom plate 501b is welded to its lower section. Further, in order to remove a thermal stress due to the welding, the container 501 is subject to heating treatment.
However, in order to obtain the bottomed container of the conventional cask 500, since the bottom plate 501b is jointed to the cylindrical body section 501a by welding, the container should be subject to the heating treatment after the welding. For this reason, there arose a problem that the manufacturing requires troublesome steps. Moreover in the Erhardt boring method, as shown in FIG. 31, a drop of a temperature in a metal forward end portion rising in the spaces 350 causes scratches and wave-shaped defects. Further, as shown in the diagrams, since a faulty form portion (FIG. 31(b)) is inevitably generated in the cylindrical end section, this portion should be eliminated by a constant amount, and thus yield is greatly lowered.
In addition, in the backward extrusion pressing method, the metal billet is formed while high friction is generated between the container and metal billet. For this reason, a lot of defects such as pockmarks and ribs are generated on an outer surface of the metal billet, and it takes a long time to remove these defects.
Further, in both the Erhardt boring method and the backward extrusion pressing method, when a dimension and a thickness of the container to be formed become large, a pressure which is required for pressing becomes extremely large. Therefore, in these methods, it was difficult to manufacture a container having large dimension and thickness. Therefore, the present invention is devised in order to solve the above-mentioned problems, and its object is to provide a container which requires less troublesome manufacturing steps or a container in which defects generated on its cylindrical end portion and its surface can be suppressed.